(i) Field of the Invention
This invention relates to a method and apparatus for the production of expanded metal mesh sheet and, more particularly, relates to a one-step method and apparatus for the production of expanded metal mesh sheet for use in lead-acid battery manufacture.
(ii) Description of the Related Art
The prior art discloses rotary methods for expanding lead strip for use in the manufacture of battery plates. Such methods employ clusters of tools arranged sequentially for preforming and slitting the strip in a first step and completion of slitting of the strip in a second step. Sequential methods have the inherent problems of synchronization of steps, such as roll-to-roll synchronization, requiring certain registering and tracking considerations.
Sequential methods use different tooling for the different steps with the result that lead strip is not xe2x80x9csymmetrically processedxe2x80x9d, in that opposite sides of the strip are not always subjected uniformly and simultaneously to the same pressures, forces, stretching, and the like. In one predominant method in the prior art, a three-shaft cluster of tooling is arranged sequentially with three different tooling devices, namely a xe2x80x9cpreformerxe2x80x9d, a xe2x80x9cpreform slitterxe2x80x9d and a xe2x80x9cslitterxe2x80x9d, such that a two-step method results. The preformer and preform slitter form the metal strip by stretching and cutting in a first step and the slitter completes the slitting in a second step.
Wires and nodes on opposite sides of the expanded strip produced by the stretching and forming according to the prior art are not uniform and are not symmetrical. The profile and shape on one side is not the mirror image of the other side resulting in a number of imperfections and defects. This becomes even more significant when higher elongation targets are desired in order to produce lighter grid electrodes for batteries.
Cominco U.S. Pat. No. 4,291,443 issued Sep. 29, 1981 and U.S. Pat. No. 4,315,356 issued Feb. 16, 1982, both included herein by reference, disclose the geometric relationship of conventional 3-shaft cluster tooling or spaced-apart roll pairs employing two sequential steps, i.e. preforming, wherein the lead strip is slit and stretched to form wires that are still solidly connected and not in a form to be pulled apart, and slitting, wherein alternate slits in the nodes are made to allow subsequent expansion to complete the process.
Cominco U.S. Pat. No. 4,297,866 issued Nov. 3, 1981, also incorporated herein by reference, discloses a sequential two-step process for the production of symmetrical slit wires deformed out of the plane of the strip having a trailing portion of the wire longer than the leading portion for improved stretchability of the wires.
Forming of the strip in a one-step process has been discounted and not achieved to date because of perceived intricacies of the grid design and physical limitations of the grid components, particularly fore-shortening and rippling of the strip. U.S. Pat. No. 1,472,769 issued Oct. 3, 1923 discloses a method and apparatus for expanding metal sheet between opposed rollers in which wire strands and bands are slit in the sheet, slit strands are returned to the plane of the sheet by flattening rolls, longitudinal corrugations are then formed in alternate series of bands in reverse directions to stretch the strands, and the sheet then laterally expanded to form a mesh. It was believed necessary to incorporate the flattening and longitudinal corrugating steps in the process for the formation of uniform meshes.
The present invention substantially overcomes the problems of the prior art and makes such one-step processing possible for the production of uniform mesh sheet particularly from ductile malleable metals such as lead and lead alloys. Uniform wire stretching, node formation and expanded mesh diamond geometry are achieved, according to the invention, in a rotary expander preferably employing cluster tooling. Wire elongation, previously limited to about 30%, can now be increased up to about 50% or more elongation for the production of light-weight batteries for use in the SLI (starting, lighting and ignition) battery industry.
A cluster tooling module utilizing one pair of opposing shafts containing identical combination former/slitter devices that slit and form all necessary grid wire components in a continuous motion is employed, resulting in no stripping or disengaging. A third tooling shaft simply adds centre and edge guiding features to the formed and slit material, for example by roll-forming the centre and perforating the edges. The resulting slit and formed lead material has uniformly stretched and shaped components on either side of the strip. The one-step method can be realized through rearrangement and retrofitting of existing tooling.
In its broad aspect, the method of the invention for forming expanded mesh sheet from a deformable strip comprises the steps of concurrently slitting and forming at least a portion of said strip contained within imperforate border portions to provide a plurality of longitudinally extending wire-like components, said components comprising elongated slit segments deformed out of the plane of the strip and alternately slit segments retained in the plane of the strip, said elongated slit segments being severed from laterally adjacent segments and said border portions and being substantially convexly shaped from the plane of the strip whereby slit segments in laterally adjacent components extend from opposite sides of the plane of the strip, and said alternately slit segments retained in the plane of the strip together define nodes extending laterally at least the width of said wire-like components across the said portion of the strip.
The apparatus of the invention for forming elongated alternately slit segments in deformable strip comprises a pair of opposed rolls each having a plurality of spaced discs having opposite side walls and circumferential, equally spaced, convexly shaped tool surfaces alternating with substantially flat surfaces, said discs having radial notches formed in the opposite sidewalls of alternate circumferential flat surfaces, whereby peripheral surfaces of opposing rolls are adapted to interact on deformable strip passing therebetween to slit and form convex segments and alternate nodes in said strip by intermeshing of said shaped tool surfaces.
The apparatus may additionally comprise a third roll having a substantially smooth peripheral surface in opposition to one of the pair of opposed rolls, whereby the third roll and a said first opposed roll are adapted to interact on deformed strip passing therebetween for roll forming the strip centre and perforating the strip edges to facilitate expansion.